1. Field of the Invention
The present invention relates to a control device that controls an electric motor drive device, which includes a DC/AC conversion section that converts a DC voltage into an AC voltage to supply the AC voltage to an AC electric motor, using a vector control method.
2. Description of the Related Art
An electric motor drive device that converts a DC voltage from a DC power source into an AC voltage through an inverter serving as a DC/AC conversion section to drive an AC electric motor is generally used. Such an electric motor drive device may perform vector control (field oriented control: FOC) to supply a sinusoidal AC voltage to coils of the AC electric motor for respective phases in order to efficiently produce a torque. In such vector control, respective currents flowing through the stator coils of the AC electric motor for three phases are subjected to coordinate conversion into two vector components in a d-axis, which is the direction of the magnetic field produced by a permanent magnet disposed in a rotor, and a q-axis, which is perpendicular to the d-axis, to perform feedback control.
Japanese Patent Publication No. JP-B-3890907 discloses a control device for an electric motor drive device configured to perform vector control for two AC electric motors by commonly using a single computation processing unit. In the control device according to Japanese Patent Publication No. JP-B-3890907, first, a current control section derives voltage command values vd′* and vq′* on the basis of current command values id* and iq* which are command values of currents for driving the AC electric motor and actual currents id and iq flowing through the AC electric motor. Next, non-interference computation is performed to derive two-phase voltage command values vd* and vq*, in which interference terms have been corrected, on the basis of the derived voltage command values vd′* and vq′*. Thereafter, a voltage control section derives three-phase voltage command values vu*, vv*, and vw* on the basis of the two-phase voltage command values vd* and vq* and a magnetic pole position qre which represents the rotational position of the rotor of the AC electric motor, and generates a control signal for controlling the inverter in accordance with the three-phase voltage command values vu*, vv*, and vw*.
In order to more precisely control the output torque and the rotational speed of the AC electric motor when driving the AC electric motor as described above, it is preferable to perform feedback control by acquiring the respective command values and actual values in as short a cycle as possible. However, performing a large amount of computation in each short cycle increases the processing load to be incurred by the computation processing unit. While the use of an additional computation processing unit or a higher-performance computation processing unit is also conceivable, such use is not preferable in terms of reducing the size and the cost of the electric motor drive device and the control device for the electric motor drive device.
In this respect, in the control device according to Japanese Patent Publication No. JP-B-3890907, a computation cycle (hereinafter occasionally referred to as a current control cycle) in which the current control section derives the voltage command values vd′* and vq′* and further derives the two-phase voltage command values vd* and vq* on the basis of the derived voltage command values vd′* and vq′* is set to be longer (specifically, twice or n times (where n is an integer)) than a computation cycle (hereinafter occasionally referred to as a voltage control cycle) in which the voltage control section derives the three-phase voltage command values vu*, vv*, and vw* and further generates the control signal for controlling the inverter on the basis of the derived three-phase voltage command values vu*, vv*, and vw*. The processing load on the computation processing unit is thus reduced by reducing the frequency of derivations of the voltage command values vd′* and vq′* and the two-phase voltage command values vd* and vq*. In the control device according to Japanese Patent Publication No. JP-B-3890907, in addition, the respective voltage command values vd′* (vd*) and vq′* (vq*) for the two AC electric motors for the next cycle are derived alternately in each current control cycle, and the control signal for controlling the inverter is output in each voltage control cycle while considering a delay time from the timing at which the magnetic pole position is taken in each of the AC electric motors. This makes it possible to obtain an output with as high resolution as possible while reducing the processing load on the computation processing unit.